Various types of wireless transmission devices, for example, smart phones, Bluetooth headphones, MP3 players and the like, have been extensively applied. Most wireless transmission devices support Bluetooth transmission of audios and other types of data. In the Bluetooth protocol (A2DP profile), one data output source (for example, a smart phone, an MP3 player or the like) may be only connected to one audio playback device, and transmit data via Bluetooth to an audio playback device for playback.
In some scenarios, the data output source needs to transmit data to two or more than two audio playback devices. For example, the stereo headphone requires that left and right-channel playback devices are both wirelessly connected to the data output source. In case of data reception, the playback device in one channel (for example, the left headphone) only acts as a primary headphone to receive the data transmitted by the data output source. Then the data is transmitted to the playback device in the other channel via Bluetooth or other non-Bluetooth transmission protocols.